1. Field of Invention
This invention relates in general to satellite navigation systems and in particular to Global Positioning System (“GPS”) receivers utilizing multiple antennas.
2. Related Art
Cellular telephony, including the use of Personal Communication System (“PCS”) devices, has become commonplace. The use of such devices to provide voice, data, and other services, such as Internet access, has provided many conveniences to cellular system users.
A current thrust in the cellular and PCS area is the integration of Global Positioning System (“GPS”) technology into cellular telephone devices and other wireless devices to provide satellite navigation. This current interest in integrating GPS with cellular telephony stems from a Federal Communications Commission (“FCC”) requirement that cellular telephones be locatable within 50 meters once an emergency call, such as a “911” call (also referred to as Enhanced 911 or “E911”) is placed by a given cellular telephone. This position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone's position. Further, GPS data can be used by the cellular user for directions, location of other locations that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular user via Internet maps or other GPS mapping techniques, etc. Such data can be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
However, since cellular telephones can travel into areas where GPS signals cannot be reliably received, augmentations to the GPS system are being researched to support the E911 and other GPS/cellular applications. GPS is increasingly being pressed into service in the cellular telephone/PDA/mobile computer application where a solution is required in areas with substantial blockage, such as inside buildings, in subway stations, and other areas where the system RF link budget is difficult to sustain communications with mobile units that travel into hostile signal reception environments such a buildings.
FIG. 1 is a block diagram of an example GPS Mobile Unit 100 that is capable of receiving GPS signals 102, 104, 106, and 108 from a plurality of GPS satellites 110, 112, 114, and 116, respectively, in a clear view environment. The GPS Mobile Unit 100 may include a GPS Receiver 118, Antenna 120, Frequency Source 122, and Position Solution Module 124. Generally, the signal Antenna 120 element is used to receive the signals 102, 104, 106, and 108 typically emanating from the multiple satellites 110, 112, 114, and 116. Examples of the Antenna 120 element may include, for example, chip antennas, wire antennas, and ceramic patch antennas. The GPS receiver 118 performs RF amplification, filtering, mixing, digitizing, and individual satellite tracking functions. The resulting GPS measurements 126 of range, timing, and Doppler etc., are extracted from the GPS receiver 118 in a manner known to the prior art. These GPS measurements 126 are then processed by the position solution module 124 that uses the GPS measurements 126 to compute user position, velocity, time etc. of the GPS Mobile Unit 100. Unfortunately, one limitation of the architecture shown in FIG. 1 is that the position solution 128 availability and accuracy is largely dependent on the received GPS signals 130 from the single source Antenna 120.
As an example of the limitations associated with a single antenna, in FIG. 2 a system diagram of a GPS Mobile Unit 200 in a multi-path environment with blockage is shown. The GPS Mobile Unit 200 may be located within structure 202 that partially blocks some to the GPS signal 204, 206, 208, 210, 212, and 214 from GPS satellites 216, 218, 220, and 222, respectively. The structure 202, as an example, may be a building, natural or man-made environmental structure (such as a canyon wall or street in a city with tall building).
As an example, a single indoor GPS antenna 223 may receive each GPS satellite 216, 218, 220, and 222 via multiple paths as the signal reflects from the blockage or possible objects in the structure 202. The received multi-path signals may result in constructive or destructive interference, with constructive interference increasing signal power and destructive interference reducing signal power. Generally, when the single GPS antenna 223 observes destructive multi-path interference (also known as “flat fading”) the resulting signal loss cannot be recovered.
Specifically as an example, GPS signals 204 and 208 are blocked by part 220 of the structure 202 while GPS signals 206, 210, 212, and 214 are passed into the interior 224 of the structure 202. However, in this example, only GPS signals 212 and 214 are directly received by GPS Mobile Unit 200 while GPS signals 206 and 210 are indirectly received by the GPS Mobile Unit 200 via multi-path GPS signals 226 and 228, respectively, that may be reflected off of an inside wall 232 of the structure 202.
Unfortunately, in previous and current teachings the use of multi-path GPS signals for satellite navigation is typically avoided because multi-path GPS signals give less accuracy and in satellite navigation, accuracy was and remains the prime goal.
As a result, there is a need for a system and method capable of receiving GPS signals and producing a location for a GPS Mobile Unit in bad propagation conditions where GPS signals are weak and blocked, and short delay, strongly interfering multi-path is dominant.